In recent years, radio communications, such as those based on portable telephones and the like, employ a modulation scheme which demonstrates a high frequency utilization efficiency and a large peak-to-average power ratio (PAPR).
In the field of radio communications, for amplifying a modulated signal including an amplitude modulated component using an AB-class amplifier which has been conventionally employed, sufficient back-off must be taken in order to maintain the linearity of an output signal. Generally, this back-off is required to be at least in the order of PAPR.
On the other hand, the AB-class amplifier exhibits a power efficiency which reaches a maximum at output saturation, and becomes lower as the back-off increases. As such, a modulated signal having larger PAPR encounters larger difficulties in increasing the power efficiency of a power amplifier.
A polar modulation type power amplifier is representative of a power amplifier for highly efficiently amplifying such a modulated signal which has large PAPR. The polar modulation type power amplifier amplifies a high-frequency modulated signal for radio communications, which is generated on the basis of polar coordinate components of amplitude and phase. Also, polar modulation type power amplifiers include one which is particularly referred to as an EER (Envelope Elimination and Restoration) type power amplifier. The EER type power amplifier is configured to substitute for an AB-class amplifier.
FIG. 1 shows the configuration of an RF (Radio Frequency) transmitter as an exemplary radio wave transmitter which comprises an associated polar modulation type power amplifier.
The RF transmitter shown in FIG. 1 comprises digital baseband unit 201, analog baseband unit 205, and EER-type power amplifier 214.
Digital baseband unit 201 generates three types of signals, i.e., a power control signal, an I-signal, and a Q-signal which are delivered to analog baseband unit 205 through power control signal output terminal 202, I-signal output terminal 203, and Q-signal output terminal 204, respectively.
In analog baseband unit 205, the I-signal delivered from I-signal output terminal 203 is applied to and converted to an analog signal by DA (Digital-to-Analog) converter 206. Likewise, the Q-signal delivered from Q-signal output terminal 204 is applied to and converted to an analog signal by DA converter 210.
The I-signal and Q-signal converted to analog signals are multiplied by signals supplied from local oscillator 208 through phase shifter 209, by mixer 207 and mixer 211, respectively. In this event, the signal supplied from phase shifter 209 to mixer 211 has a phase delayed by 90° from the signal supplied from phase shifter 209 to mixer 207.
The output signal of mixer 207 and the output signal of mixer 211 are added by adder 212 to generate a high-frequency modulated signal. The high-frequency modulated signal delivered from adder 212 is amplified by variable gain amplifier 213, and then delivered to EER-type power amplifier 214. In this event, the gain of variable gain amplifier 213 is varied in accordance with a power control signal delivered from power control signal output terminal 202.
In EER-type power amplifier 214, the high-frequency modulated signal delivered from variable gain amplifier 213 is applied to envelope detector 215 and limiter 219. Envelope detector 215 extracts an envelope signal from the high frequency modulated signal input thereto. The envelope signal extracted by envelope detector 215 is linearly amplified in an amplification path which is provided with AD (Analog-to-Digital) converter 216, switching amplifier 217, and low-pass filter 218. Limiter 219 extracts a phase modulated signal, which presents a substantially uniform envelope, from the high-frequency modulated signal input thereto, and delivers the phase modulated signal to high-frequency power amplifier 220. High-frequency power amplifier 220 is supplied with the envelope signal delivered from low-pass filter 218 as a power supply, and amplifies the phase modulated signal delivered from limiter 219 by multiplying the same by the power supply. The thus amplitude modulated output signal is delivered from signal output terminal 221.
EER-type power amplifier 214 can increase the power efficiency because it can employ switching amplifier 217 which is highly efficient in the amplification of the envelope signal, and because the multiplication processing can be highly efficiently performed in high-frequency power amplifier 220.
A signal band handled by envelope detector 215 is similar to a signal band for the output signal of variable gain amplifier 213, and typically ranges approximately from several hundreds of kHz to several tens of MHz. Accordingly, the envelope signal can be amplified by a D-class amplifier or the like, which comprises AD converter 216 for generating a bit stream signal such as PDM (Pulse Density Modulation) or the like, switching amplifier 217, and low-pass filter 218, ideally without causing a power loss.
Meanwhile, high-frequency power amplifier 220 is operating in a saturation region with the output signal of low-pass filter 218 which is being supplied as a power supply. Generally, high-frequency power amplifier 220 is characterized by operating at the highest power efficiency when the output is saturated.
With the foregoing configuration, the power efficiency of EER-type power amplifier 214 is given by the product of the power efficiency of switching amplifier 217 with the power efficiency of high-frequency power amplifier 220, and theoretically, high-frequency power amplifier 220 provides the highest efficiency at all times.
Alternatively, polar modulation type power amplifiers have been proposed for performing amplitude modulation in different ways, other than EER-type power amplifier 214 shown in FIG. 1. As an example, FIG. 2 shows the configuration of a polar modulation type power amplifier described in Patent Document 1.
The polar modulation type power amplifier shown in FIG. 2 performs amplitude modulation by switching the number of saturated amplification units 304 which are set to an operable state (is turned on) among a plurality of saturated amplification units 304.
First, local oscillator 303 applies a phase modulated signal to each of the plurality of saturated amplification units 304. On the other hand, an amplitude modulated signal is applied from modulated signal input terminal 301, and is converted to a control signal for saturated amplification units 304 by amplitude controller 306. The control signal from amplitude controller 306 determines whether each of a plurality of saturated amplification units 304, arranged in parallel, should be set into an operable state or a sleep state (off-state). The phase modulated signals delivered from saturated amplification units 304 in the operable state are combined by output combiner circuit 305, and delivered from output terminal 302.
Here, the modulated signal delivered from output terminal 302 has an amplitude which is a sum total of the amplitudes of the phase modulated signals delivered from saturated amplification units 304. Accordingly, the amplitude modulation can be performed by varying the number of saturated amplification units 304 which are in the operable state.
However, since the polar modulation type power amplifiers shown in FIGS. 1 and 2 convert an envelope signal to a time discrete signal which is quantified through AD conversion, quantization noise occurs. In the polar modulation type amplifier, the quantization noise has a magnitude which is substantially uniform irrespective of output power, so that the output signal deteriorates in SNR particularly when the output power decreases more from a maximum power.
In the polar modulation type power amplifiers shown in FIGS. 1 and 2, the output signal varies in SNR depending on the output power for causes which are attributable to basic characteristics of the AD converter for use in amplification of the envelope signal. FIG. 3 shows the relationship between an input power and SNR of an output signal in an ideal AD converter.
As shown in FIG. 3, in an ideal AD converter, SNR (dB) of an output signal can be represented by a first-order function of input power (dBm) within the range of output saturation. This fact is shown, for example, in Non-Patent Document 1, Non-Patent Document 2, and the like.
For the reason set forth above, when an envelope signal applied to an AD converter as an input signal has a lower average power, an envelope signal delivered from the AD converter will deteriorate in SNR.
For example, in W-CDMA based communications, the radio wave strength is adjusted in accordance with the distance between a base station and a portable terminal. Accordingly, a W-CDMA based radio wave transmitter requires a circuit for controlling the output power of a polar modulation type power amplifier.
In the radio wave transmitter shown in FIG. 1, the output power of EER-type power amplifier 214 is adjusted by variable gain amplifier 213 which is positioned antecedent to AD converter 216. Consequently, since AD converter 216 is applied with an envelope signal with a varying average power, SNR will vary in the envelope signal amplification path.
Likewise, SNR also varies in the polar modulation type power amplifier shown in FIG. 2. For controlling the output power with the polar modulation type power amplifier shown in FIG. 2, it is necessary to vary the number of saturated amplification units 304 which are controlled to be in operable state by amplitude controller 306. Amplitude controller 306 plays the same role as AD converter 206 shown in FIG. 1, and the number of bits representative of the magnitude of an envelope signal is represented by the number of saturated amplification units 304 which are in operable state. Accordingly, the relationship between the input power of the envelope signal and the SNR in the polar modulation type power amplifier shown in FIG. 2 is the same as that shown in FIG. 3, so that the envelope signal varies in SNR due to variations in average power of the output signal.
As described above, in a power amplifier used in a radio wave transmitter, a change in the average power of an output signal causes the SNR to vary on an envelope signal amplification path, resulting in variations in SNR of the output signal. Particularly, the output signal tends to deteriorate in SNR when a power amplifier generates an output signal with reduced average power.
From the foregoing, the power amplifier has a challenge in maintaining SNR of the output signal substantially constant irrespective of the average power of the output signal.    Patent Document 1: JP2005-86673A (FIG. 4).    Non-Patent Document 1: “Systematic Design of Sigma-Delta Analog-to-Digital Converters,” authored by Ovidiu Bajdechi and Johan H. Huijsing, Kluwer Academic Publishers, p. 16, FIG. 2.6.    Non-Patent Document 2: “Bandpass Sigma Delta Modulators,” authored by Jurgen van Engelen and Rudy van de Plassche, Kluwer Academic Publishers, p. 47, FIG. 4.7.